Abstract

Two railroad curve transition spiral shapes are compared with respect to their dynamic performance: the traditional linear shape and an improved shape intended to provide optimal dynamic performance. The improved shape is attractive because it gives much better dynamic performance and flows from an improved way of thinking about spirals. The improved spiral design method and spiral shape to be evaluated are discussed, then results from simulating the operation of a rail vehicle over an improved spiral and over the corresponding traditional spiral are compared. The simulations were done using the NUCARS simulation program and a model of the Amtrak Acela passenger car. The simulated motion of the vehicle over the traditional spiral exhibits undesirable fluctuations of two types with different strengths and distinct causes. The weaker fluctuations are the result of impacts of wheel flanges against the outside rail partway through the spiral. These impacts might be softened or eliminated by use of a truck design that could steer more effectively than the design that was simulated (or by making spirals much longer). They are not much affected by the kind of spiral geometry used. The stronger fluctuations are caused by the shape of the traditional spiral and particularly by the abruptness with which superelevation ramp angle changes at each end. The improved spiral geometry eliminates these fluctuations almost completely and thereby will improve ride comfort and may reduce the cost of maintaining spiral track alignment.